Use datetime instead of str for all model timestamps

[mesozoic]

This branch switches Model.created_time and all timestamps in schema/webhook/comment/etc. models to use datetime.datetime instead of the str representation that comes back from the API.

The approach is a bit wonky – inspect pydantic field metadata, check types against fields and input, selectively convert data – but it works, and doesn't require us to do anything special per field except declare its type differently than what Airtable will hand back in the API.

This is certainly a breaking change, so I'd include it in a 3.0 release.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (eef7b6f) to head (a4da1c6).
Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #352   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           25        24    -1     
  Lines         2313      2334   +21     
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+ Hits          2313      2334   +21     

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